The present invention relates to a stabilization of laser oscillation wavelength.
FIG. 5 is a schematic illustration of such a conventional system for stabilizing laser wavelength as shown in IEEE Journal Quantum Electronics QE-14 ('78) 17. In FIG. 5, a laser oscillator 1 is equipped with a component for changing wavelength of laser oscillation beam 2 which is directed through a reflection mirror to a Fabry-Perot etalon 3. An output laser beam from the Fabry-Perot etalon 3 is detected by an optical detector 4 an output of which is supplied to a servo mechanism 5 to control wavelength of laser output of the oscillator 1.
Wavelength of laser beam from the laser oscillator 1 depends upon conditions of an optical resonator of the oscillator and, in the shown example, laser wavelength can be selected by changing an optical length of the resonator. However, a wavelength selected in this manner is hardly be stabilized due to thermal deformation or vibration of the resonator. In order to solve this problem, the shown system employs the Fabry-Perot etalon which ia a high resolution spectroscope to detect intensity of laser beam passed therethrough by means of the optical detector 4 upon which the servo mechanism 5 is actuated to stabilize laser wavelength. That is, the Fabry-Perot etalon is composed of a pair of mirrors having high flatness and disposed oppositely with a gap d therebetween and, by passing light therethrough at an angle .theta. with respect to mirror surfaces, it becomes to have a specific wavelength having center wavelength .lambda. m represented as follows: EQU .lambda.m=(2nd cos .theta.)/m.sup.2
where n is a refraction index of the gap and m is an integer. By using such Fabry-Perot etalon having high resolution, an intensity of .theta. m in wavelength distribution of laser oscillation is obtained.
FIG. 6 shows a curve (a) which shows a variation of resonator distance and peaked curves (b) which show corresponding change of beam intensity caused by a change of beam wavelength .theta. which results in beam intensity corresponding to the center wavelength .theta. m of the Fabry-Perot etalon. That is, the curve (b) show spectrum distribution of oscillation wavelength of the oscillator 1. A dip portion around the peak intensity of each curve (b) is called as ram dip.
When the resonator length is increased gradually within a section (c) corresponding to the ram dip, intensity of beam passed through the Fabry-Perot etalon decreases firstly and then starts to increase at a center wavelength of the dip. Therefore, by using the so-called "Stabilization using Phase Detection" in which the resonator length is changed by the servo mechanism 5, while detecting a direction of change of intensity of beam passed through the Fabry-Perot etalon, in such a way that wavelength is concentrated to a point at which the changing direction of beam intensity is changed, it is possible to fix the oscillation wavelength .lambda. to the center wavelength .lambda. m of the Fabry-Perot etalon.
The center wavelength .lambda. m of the Fabry-Perot etalon which provides a reference for wavelength stabilization tends to drift due to an unintended physical change or the Fabry-Perot etalon, such as change of mirror gap, change of environmental tempe-rature and/or change of pressure. In order to respond such drift of the center wavelength of the Fabry-Perot etalon, the system shown in FIG. 5 uses another laser 6 which is preliminarily stabilized in another way and another optical detector 8 for detecting intensity of light from the laser 6 passed through the Fabry-Perot etalon 3. The Fabry-perot etalon is designed such that it functions to provide a wave-length selector for laser wavelength from the laser 6 in such a way that an intensity of light from the Fabry-Perot etalon is substantially reduced when the selected wavelength of the Fabry-Perot etalon for the laser 6 is drifted by even a small amount. That is, drift of the Fabry-Perot etalon can be detected by monitoring intensity of light from the laser 6 and passed through the Fabry-Perot etalon by means of the optical detector 8. Such variation of light intensity detected by the optical detector 8 is fedback to the Fabry-Perot etalon by the servo mechanism 7 to stabilize the Fabry-Perot etalon.
In order to determine the direction of change of intensity of beam passed through the Fabry-Perot etalon, it is necessary to have an enough time to scan wavelength and to have a stable output of the Fabry-Perot etalon for at least such time. Therefore, the center wave-length must be fixed at the ram dip due to the control method and it is impossible to tune it to another wavelength. Further, since there is a rest period of the laser 1, it becomes impossible to shift the wavelength center back to the original value when it is shifted out of the region (c).